Formation of Noble Metal Phases (Pt, Pd, Rh, Ru, Ir, Au, Ag) in the Process of Fractional Crystallization of the CuFeS2 Melt

Abstract

The quasi-equilibrium directional crystallization of the melt composition (at. %): Cu 24.998, Fe 25.001, S 49.983, with Ag 0.002, Pd 0.003, Ru 0.004, Rh 0.006, and Au, Pt, Ir (each as 0.001) was carried out. The crystallized cylindrical ingot consisted of two primary zones and three secondary zones with different chemical and phase compositions. The compositions of the primary zones corresponded to high-temperature intermediate solid solution (zone I) and liquid enriched in sulfur (zone II). The compositions of the secondary zones corresponded to low-temperature intermediate solid solution and chalcopyrite (zone Ia), the same intermediate solid solution with chalcopyrite and bornite (zone Ib), and again with bornite, chalcocite, and idaite (zone II). We plotted the distribution curves of Fe, Cu, and S along the ingot, calculated the distribution coefficients of the components during directional crystallization, and clearly showed that, from the initial stoichiometric composition CuFeS2, the intermediate solid solution enriched in Fe and depleted in S is crystallized. Based on the data of directional crystallization and thermal analysis, a cross section was constructed in the intermediate solid solution-sulfide melt region of the Cu-Fe-S system. With solubility in the solid Cu-Fe sulfides lying below detection limit of scanning electron microscopy/energy-dispersive X-ray spectrometry (SEM/EDS), noble elements occurred as individual phases of a size more often <10 µm. They were identified as Ag, RuS2, PdS, Au* (an Au based alloy), (Rh, Ir, Ru)3S8, (Rh, Ir)3S8, Rh3S8, and (Cu, Fe)~2(Pt, Rh)1S~5 phases by electron microprobe. Based on ab initio calculations of crystal structure, electronic band structure, and lattice dynamics of idealized laurite RuS2 phase and the idealized Ir3S8, Rh3S8, and Ru3S8 phases, the interpretation of Raman spectrum of the cation-mixed (Ru, Rh, Ir)S2 sulfide was presented for the first time.